The fiber-based packaging material of product packages, such as packing paper or board, is usually provided with a polymeric coating that makes the package tight and by means of which the package can be closed by heat sealing. Multi-layer coatings can comprise an inner EVOH, PET or polyamide layer that provides the material with an effective barrier to water vapor and oxygen, and an outer poly-olefin layer that makes the material heat-sealable. One disadvantage of the said widely-used coating polymers is, however, that they are not biodegradable.
Polylactide (PLA), which has reasonably good moisture and gas barrier properties that are adequate to many applications, has been used as the coating polymer of biodegradable packaging material; however, its use involves a number of problems. Polylactide as such is stiff and fragile, requiring a high extrusion temperature and a fairly large layer thickness to stick to the fiber substrate of the packaging material. Because of the high temperature, polylactide runs the risk of breaking, and in extrusion, the edges of a molten web tend to tear and pin holes easily remain in the extruded layer.
As a solution to said problems, the specification FI-112624 B (EP-1094944 B1, respectively) discloses an inner adhesion layer, which is co-extruded together with an outer polylactide layer and which consists of a biodegradable polymer, examples of which, according to the specification, include some commercial copolyesters, cellulose esters, and polyester amides. They facilitate the extrusion of polylactide and provide adhesion that prevents the coating from peeling off the fiber substrate.
Another problem with the use of polylactide in the outer coating layer of the packaging material is its fairly high melting point, and the resulting poor heat sealing ability. As an improvement to this, the specification US-2002/0065345 A1 discloses a biodegradable aliphatic polyester that is blended with polylactide, its portion in the mixture being at least 9%, and a tackifier, its portion in the mixture being at least 1%. As suitable aliphatic polyesters, the publication mentions polycaprolactone (PLC) and polybutylene succinate adipate (PBSA). According to the patent specification, the mixture can be extruded into a film, which can be axially or biaxially stretched and which can be attached to the fiber substrate by lamination. As a result, polymer-coated biodegradable packaging material is obtained, which has a considerably improved heat sealing ability.
The specification of US 2005/0192410 A1 describes polylactide films and coatings, wherein the processability of polylactide is improved by blending with it 10-40 weight-% of polycaprolactone and 5-10 weight-% of mineral particles. According to the specification, the mixture can be used in extrusion coating, but there is no reference to its adhesion to the fiber substrate or to its heat sealing ability in the specification. Instead, the specification describes the intermediate layers between the carrier and the PLA-based coating layer or the top layers that come on top of the PLA layer compare paragraph [0039] in the specification.
The specification of US 2007/0259195 A1 describes polylactide-based films, which contain, blended therewith, 0.1-10 weight-% of biodegradable polymeric additive, the purpose of which is to increase the crystallinity of the polylactide, improving its heat resistance. As examples of such additives, the specification presents FEPOL 2040 marketed by Far Eastern Textile, Taiwan, and Ecoflex marketed by BASF, both of which comprise polybutylene adipate terephthalate (PBAT). According to the specification, the mixtures can be extruded onto the fiber substrate in a conventional manner, but there is no reference to the adhesion of the mixture to the substrate or to the heat sealing ability of the coating thus obtained. In the specification, the intended improved heat resistance of PLA does not, however, refer to an improvement of the heat sealing ability but rather to its weakening.